I would love to have it longer but I don't want to give those twunts at customs an excuse to keep it :-/

so a fully self contained cartridge relying on this principle is a viable option, right ?

A cartridge fed ballistic knife?!

hehehe

The reason this works well is that the knife is quite heavy, so the piston doesn't accelerate much and stopping it is relatively easy. The delrin piston seals against the delrin barrel, to the extent that I have to let the gasses out from the schrader. The chamber also gets extremely hot.